1. Field of the Invention
The present invention relates to a DC power source apparatus used for an electronic or electrical equipment and, more particularly, to a polyphase input DC power source apparatus applied to a medical equipment such as an x-ray CT or MRI apparatus and connected to a polyphase AC power source to improve an input power factor.
2. Description of the Related Art
FIG. 1 is a block diagram showing the arrangement of a capacitor-input DC power source apparatus. A single-phase AC power source 1 is connected to a single-phase rectifying circuit 2 formed of bridge-connected diodes D1 to D4. A capacitor 3 is connected between the output terminals of the rectifying circuit 2. A load 4 is connected to the two terminals of the capacitor 3. The power source 1 is connected to the rectifying circuit 2 through impedances zp.
In the power source circuit shown in FIG. 1, assume that an input voltage to the rectifying circuit 2 is Vin, an input current is Iin, an input rectified voltage is Vd, an input rectified current is Id, and a terminal voltage of the capacitor 3 (output voltage from the rectifying circuit 2) is Vc. Only when the input rectified voltage Vd is higher than the terminal voltage Vc of the capacitor 3, the input current Iin having a pulse waveform flows, as shown in FIG. 2.
Because of this pulse-like input current Iin, the power factor of this power source circuit is as low as 0.5 to 0.6. The peak value of the pulse-like input current is relatively large. For this reason, a distribution equipment including the AC power source 1 need to have a power capacity much larger than the actual power consumption. The impedances Zp connected between the AC power source 1 and the rectifying circuit 2 temporarily cause a large voltage drop when the large input current Iin flows, resulting in distortion of the waveform of the input voltage Vin. Referring to FIG. 2, a broken line represents an input voltage waveform without distortion, and a solid line represents a distorted voltage waveform.
In addition, the Fourier-transformed pulse-like input current Iin includes a lot of harmonic components, as shown in FIG. 3.
Such distortion of the input voltage waveform or harmonic components of the input current affects, as a noise source, other electrical equipments connected to the AC power source 1.
In order to prevent distortion of the voltage waveform and decrease the harmonic components of the input current (i.e., improve the power factor), as shown in FIG. 4, it is known that the waveform of the input current Iin is effectively controlled in proportion to the waveform of the input voltage Vin. As a result, as shown in FIG. 5, it is known that the harmonic components are eliminated from the waveform of the Fourier-transformed input current Iin to effectively obtain only fundamental wave components.
Such a DC power source apparatus which controls the input current waveform to improve the power factor, decrease harmonic components, and prevent distortion of a voltage waveform is described in "DENSHI GIJUTSU" March 1990 (special enlarged issue), pp. 90-99 as an active smoothing filter control IC (150-w step-up DC/DC converter). FIG. 6 is a block diagram showing the arrangement of this DC power source apparatus. As in the conventional apparatus in FIG. 1, a rectifying circuit 2 is connected to a single-phase AC power source 1. A step-up chopper circuit 5 formed of a choke coil 6 and a switching element 7 is connected between the output terminals of the rectifying circuit 2. The connection point between the choke coil 6 and the switching element 7 is connected to the anode of a diode D5. The cathode of the diode D5 is connected to one terminal of a capacitor 3. The other terminal of the capacitor 3 is connected to the other terminal of the switching element 7 (one terminal thereof is connected to the choke coil 6). A load 4 is connected to the two terminals of the capacitor 3.
A controller 8 is connected to the chopper circuit 5. The controller 8 monitors an input rectified volt age Vin, an output voltage Vout, and an input rectified current Id detected by a current sensor 9 to ON/OFF-control the switching element 7 on the basis of these monitored values.
FIG. 7 is a graph showing the relationship between the ON/OFF operation of the switching element 7 of the DC power source apparatus, the input rectified voltage Vd, and the input rectified current Id.
At time t=0, Vd is lower than Vout, so that no input rectified current Id flows if the switching element 7 is off. When the controller 8 turns the switching element 7 on, the current flows in the choke coil 6. At this time, the input rectified current Id flows through a path indicated by a solid line or a broken line in FIG. 8.
When the switching element 7 is turned off at time t=t1, the energy accumulated in the choke coil 6 is discharged and transferred to the capacitor 3 through the diode D5. At this time, the input rectified current Id decreases, as shown in FIG. 7. A path through which the input rectified current Id flows is indicated by a solid line or a broken line in FIG. 9.
when the switching element 7 is turned on again at time t=t2, the current flows in the choke coil 6 as at time t=0, as shown in FIG. 8, and the energy is accumulated. At this time, the input rectified current Id increases, as shown in FIG. 7.
When the switching element 7 is then turned off, the energy accumulated in the choke coil 6 is discharged as at time t=t1, as shown in FIG. 9, and transferred to the capacitor 3 through the diode D5.
As described above, the switching element 7 is repeatedly turned on/off to cause the input rectified current Id and input current Iin to constantly flow. The magnitude or waveform of the input rectified current Id and input current Iin can be controlled by controlling the ON/OFF time ratio of the switching element 7.
Therefore, when the controller 8 monitors the input rectified voltage Vd, the output voltage Vout, and the input rectified current Id to control the switching element 7 such that the input rectified current Id is proportional to the input rectified voltage Vd, a high power factor can be obtained.
As the controller 8 for performing such switching control, for example, an ML4821 (tradename) is commercially available from Microlinear.
For a low-power equipment, a single-phase AC input scheme is used for a DC power source. However, in an application to a high-power equipment, it is frequently to use a three-phase AC power source instead of the single-phase AC power source 1, and convert a three-phase AC voltage into a DC voltage.
Such a three-phase AC-DC power source is described in "Polyphase Rectifier with Constant-Energy Modulation Technique", Ryoji Saito, Yoshio Suzuki, and Kazuhiro Seo, Proceeding of INTELEC 82, pp. 321-326, in which three single-phase input DC power source apparatuses are connected in parallel. A three-phase AC-DC power source circuit in which three single-phase input power source circuits shown in FIG. 6 are connected in parallel on the basis of the above concept is shown in FIG. 10.
Secondary coils 11a to 11c, each corresponding to one phase of a three-phase transformer 11, are connected to rectifying circuits 12a to 12c formed of bridge-connected diodes, respectively.
Step-up chopper circuits 15a to 15c formed of choke coils 16a to 16c and switching elements 17a to 17c are connected between the output terminals of the rectifying circuits 12a to 12c. The connection points between the choke coils 16a to 16c and the switching elements 17a to 17c are commonly connected to a capacitor 13 through diodes D5a to D5c. A load 14 is connected in parallel to the capacitor 13.
Controllers 18a to 18c for controlling the switching elements 17a to 17c monitor an input rectified voltage Vd, an output voltage rout, and an input rectified current Id to ON/OFF-control the switching elements 17a to 17c, respectively, on the basis of these monitored values such that the waveform of the input rectified current Id is proportional to that of the input rectified voltage Vd.
The above power source apparatuses are related to an apparatus for obtaining an output voltage of a single polarity. However, a standard electrical equipment frequently requires both positive and negative outputs as DC outputs. Therefore, a DC power source apparatus as shown in FIG. 11 is considered to obtain both positive and negative outputs by using the arrangement in FIG. 10.
Secondary coils 21a to 21c of a three-phase transformer 21 are connected to rectifying circuits 22a to 22c through low-pass filters 20a to 20c, respectively.
Of the positive and negative output terminals of the rectifying circuits 22a to 22c, the positive output terminals are connected to step-up chopper circuits 25a to 25c formed of choke coils 26a to 26c and switching elements 27a to 27c, respectively. The negative output terminals are connected to step-up chopper circuits 25d to 25f formed of choke coils 25d to 26f and switching elements 27d to 27f, respectively.
The output terminals of the step-up chopper circuits 25a to 25c on the positive side are commonly connected to a capacitor 23a through diodes D25a to D25c. The output terminals of the step-up chopper circuits 25d to 25f on the negative side are commonly connected to a capacitor 23b through diodes D25d to D25f.
A common line (COM) is connected to the intermediate taps of the secondary coils 21a to 21c of the three-phase transformer 21. The common line (COM) is connected to one terminal of each of the switching elements of the step-up chopper circuits 25a to 25f (the other terminal is connected to a corresponding one of the connection points with the choke coils).
The switching elements is ON/OFF-controlled on the basis of monitored values of an input rectified voltage Vd, an output voltage Vout, and an input rectified current Id by controllers 18a to 18f, respectively.
However, in the three-phase power source for obtaining both positive and negative outputs as described above, the number of components such that the step-up chopper circuits 25a to 25f formed of the choke coils 26a to 26f and the switching elements 27a to 27f, or the diodes D25a to D25f are required. In addition, the controller 18a to 18f are required to ON/OFF-control the switching elements 27a to 27f to complicate the circuit arrangement. Further, the cost is increased in accordance with an increase in number of components such as choke coils, switching elements, diodes, and controllers.
As described above, when a DC power source apparatus for obtaining both positive and negative outputs and improving the power factor is applied to a three-phase power source, the number of components such as choke coils constituting step-up circuits is increased, and at the same time, the number of controllers is increased, resulting in a complicated and expensive circuit arrangement.